The whole drivebelt end needs to come apart, basically. All accessories (ps, water pump, tensioner, alt) need to come off, and it can be cramped working at that angle, but it can be done with the engine still in the car. The timing chain cover and the crank/cam sprockets need to come off too. It might just be a clogged oil passage. Did you check the rear valve cover for any oil spray? If not, you might have a bad pump. If so, suspect a blockage on the left (front) end of the engine.